1. Field of the Invention
This invention relates to a coordinate input apparatus and, more particularly, to a coordinate input apparatus in which elastic-wave vibration that has been entered from a vibrating pen is sensed by a plurality of vibration sensors provided on a vibration transmitting tablet, and the coordinates of a point at which vibration is entered by the vibrating pen is sensed based upon a time delay in the elastic-wave input to the vibration transmitting tablet from the vibrating pen.
2. Description of the Related Art
Ordinarily, in an apparatus of this type, the time delays required for the vibration generated by the vibrating pen to reach vibration sensors provided at prescribed positions of the vibration transmitting tablet are measured, and the distances from the vibrating pen to the vibration sensors and, hence, the coordinates designated by the vibrating pen, are calculated based upon the measured values.
Vibration detected by a vibration sensor contains, in addition to a direct-wave component that reaches the vibration sensor directly upon being generated by the vibrating pen, a reflected-wave component that reaches the vibration sensor upon being reflected at, say, the end face of the vibration transmitting tablet. Components other than the direct-wave component needed for coordinate calculation are unnecessary components and result in erroneous calculation of coordinates. For this reason, it is required that the effects of the unnecessary reflected-wave component be reduced. In order to suppress the level of reflected waves, an expedient used in the art is to attenuate the reflected waves by affixing a vibration suppressor in the proximity of the end face of the vibration transmitting tablet. Despite this arrangement, however, there are cases in which the effects of reflected waves appear when the external configuration of the coordinate input unit is made small in size. Specifically, when the apparatus is made compact, the effective area of the apparatus approaches the vibration suppressor. Consequently, when vibration generated by the vibrating pen is reflected at the boundary of the vibration suppressor and the resulting reflected waves arrive at a vibration sensor, these reflected waves exhibit a large angle of incidence with respect to the vibration suppressor. When the angle of incidence approaches 90.degree., the reflectivity at the boundary where the vibration suppressor is affixed approaches 1. Accordingly, there are instances where the level of the reflected waves is enlarged, as a result of which coordinates are sensed erroneously owing to the effects of the reflected waves.
The greater the distance from the point designated by the vibrating pen to the particular sensor, and the closer this point is to the vibration suppressor within the confines of the effective area, the greater the angle of incidence. In such cases the aforementioned problem becomes more pronounced.
In order to solve this problem, an arrangement has been provided in which the region of a pen-designated position in an effective area for coordinate designation is discriminated based upon a delay time necessary for elastic-wave to reach a vibration sensor from a vibration input pen by which the vibration has been applied to a vibration transmitting tablet, and coordinate calculation is performed, on the basis of the discriminated region, without using sensors readily susceptible to the effects of reflected waves because of a large angle of incidence of the reflected waves with respect to the vibration suppressor, wherein these waves reach vibration sensors owing to reflection at the boundary with the vibration suppressor.
FIG. 7 is a diagram for describing coordinate calculation in each of four (first through fourth) quadrants obtained by subdividing a coordinate input surface, which is based upon X, Y coordinates.
In this example of the prior art, a set of sensors is decided in dependence upon the particular quadrant in which a point designated by a vibrating pen resides. On the basis of data obtained by sensors decided, the x and y coordinates are calculated using one set of sensors for the x coordinate and a different set of sensors for the y coordinate.
More specifically, in a case where an input is made in, say, the first quadrant, a sensor 6a is most susceptible to the effects of waves reflected at the boundary where the vibration suppressor is attached. Accordingly, coordinate calculation is performed based upon signals other than the signal from sensor 6a. For example, in this instance the x coordinate is calculated based upon output signals from sensors 6c, 6d and the y coordinate is calculated based upon output signals from sensors 6b, 6d. Next, when the point at which an input is made shifts to the second quadrant, sensor 6b is most susceptible to the effects of reflected waves. Therefore, the x coordinate is calculated using the same set of sensors at in the first quadrant but the y coordinate is now calculated using sensors 6a, 6c. When the point at which an input is made crosses a boundary between quadrants, there are cases where the amount of error contained in the coordinate values calculated on the basis of sensors 6a, 6c will differ from the amount of error contained in the coordinate values calculated on the basis of sensors 6b, 6d. This can result in steps between quadrants, as shown in FIG. 9.
Thus, in an arrangement in which vibration sensors are selected region by region in order to perform coordinate calculation, a discontinuity in coordinate output can occur at portions where one region changes to another, i.e., where there is a changeover in the set of sensors selected. The discontinuity is caused by a shift in constants, such as a shift in the speed of vibratory propagation with regard to data from the selected vibration sensors, or a shift in the positions of the sensors.